THE EFFECT OF STIMULUS AREA ON VISUAL INTENSITY THRESHOLD

56 Pages·02.341 MB·English

by  AUSTIN, GEORGE ALLEN

THE EFFECT OF STIMULUS AREA
ON VISUAL INTENSITY THRESHOLD
by
George A. Austin
A dissertation stibmitted in partial fulfillm ent
of the requirements for the degree of
Doctor of Philosophy in the
University of Michigan
1951
Committee in charge:
A ssistant Professor H. Richard Blackwell, Chairman
Professor Donald G. Marquis
Associate Professor Carl R. Brown
Dr. Wilfred M. Kincaid
Dr. Donald W. Lauer
ACKNOVJLSDGEMENTS
The author was very fortunate to have access to the excellent
research facilities of the Vision Research Laboratory#, University of
Michigan, and the pleasant cooperation of its entire personnel in
carrying out the present series of experiments. The greatest debt is to
Dr. H. Richard Blackwell, director of the laboratory, under whose guidance
the experiments were planned and with whose help the data were collected
and interpreted. Mathematical analysis of the data was carried out under
the general direction of Dr. W. M. Kincaid, who also gave freely of his
help at a ll stages of the investigation.
Mr, James G. Ohmart performed numerous mechanical services.
Mrs. Kenneth A. Stone and Mrs. H. R. Hiltner did the secretarial work.
Special mention should be made of Mr. John H. Taylor, who acted as one of
the subjects and who made the photometric determinations.
Final acknowledgment is made to Dr. D. G. Marquis, who can never
be repaid for his general advice, support and encouragement in my graduate
training.
#Operating tinder contract with the Office of Naval Research, Navy Department
TABLE OF CONTENTS
Page
LIST OF TABLES............................................................................ iv
LIST OF ILLUSTRATIONS.......................................................... v
Chapter
I. INTRODUCTION....................................................................... 1
II. EXPERIMENTAL PROCEDURES...................................................... 4
A. Stimulus C onditions................................................. 4
B. Method of jStimulus Presentation....................... 8
C. Subjects^....................................................................... 13
D. Photometry........................................................................ 13
S. Analysis of Data.......................................................... 15
III. EXPERIMENT I. ORIENTATION LIGHT SEPARATION . . . 17
IV. EXPERT PUT II. SENSITIVITY MAP CF FOVEA . . . . 26
V. EXPERIMENT III. THE AREA EFFECT..................................... 32
VI. DISCUSSION...................................................................................... 38
A. Theories of the Area E ffe c t................................ 38
B. Interpretation of R esu lts.................................... 43
VII. SUMMARY........................................................................... 48
V III. BIBLIOGRAPHY............................... 49
iii
LIST OF TABLES
Table Page
1. Spectral Transmissions................................................ 6
2. Psychometric Data, Experiment IA.............................................. 20
3. Psychometric Data, Experiment IB.............................................. 22
4. Psychometric Data, Experiment I I ........................... 28
5. Psychometric ^ata, Experiment I I I ........................... 33
6. Calculations from Smoothed Curve, Experiment III. . . 44
iv
LIST OF ILLUSTRATIONS
Figure Page
1. Relative Visual Effect, Test Stimulus and
Orientation Lights........................................................................... 7
2. Floor Plan of Laboratory, Schematic ................................... 11
3. Stimulus Presentation Apparatus, Schematic..........................12
4. Practice Effect, Experiment I .....................................................IS
5. Thresholds, Experiment IA .........................................................21
6. Relative Thresholds, Experiment IB....................................... 23
7. Thresholds, Experiment II, JHT.....................................................29
S. Thresholds, Experiment II, GAA.................................. 30
9. Thresholds, Experiment III. . ................................................34
10. "Summation Function," Experiment III................................... 45
v
I
I. INTRODUCTION
Early in the history of visual experimentation it was discovered
that threshold brightness is lower for large stim uli than it is for small
stim uli, for given brightness of background. This phenomenon is called
the "area effect," The classic expression of the area effect is Ricco’s
law, which states that for stimuli whose images fa ll on the fovea, the
relationship is an inverse one: A xAB * constant, where A represents
stimulus area and aB represents threshold brightness. Later investigators
reported the relationship not to be exactly inverse, but rather in the
form x AB = constant, k my be called the "coefficient of area effect,"
When the logarithm of A is plotted against the logarithm of B according
to this more general form of the expression, the result is a straight line
with a slope of -*k.
It v/as a source of some confusion, however, that each investigator
reported a different value of k. It finally became clear from the data
of Graham (10) and Blackwell (3) that the graphical plot is not linear but
curved, with asymptotes at A x AB - constant (Ricco’s law) for small
stimulus areas, and aB - constant (absence of the area effect) for very
large stimulus areas. The object of the present investigation is to deter­
mine, in so far as possible, vhether the area effect as it occurs in the
fovea can be accounted for on the basis of optical effects such as optical
blurring and optical smearing due to eye movements of a ll kinds. If the
area effect cannot be accounted for on this basis, then we must refer it to
specifically sensory mechanisms. „Fe must avoid the temptation to refer the
area effect to sensory mechanisms until we have clearly ascertained the
influence of the optical effects mentioned above.
In order to insure that the test stim uli were imaged on the fovea,
it was necessary to employ lights for orientation and fixation. Since
the area effect consists of the influence of stimulating one part of the
retina on the sensitivity of a spatially adjacent part of the retina, it
was necessary to ascertain the extent to which the lights used for fixa­
tion and orientation influenced the threshold of the test stimulus. The
influence should either be zero or a constant effect. Accordingly, Experi­
ment I is devoted to finding the smallest distance between test stimuli
and the orientation lights, such that larger distances ("separations")
produce no further decrease in the threshold. It was found that if the
orientation lights were separated by 33 minutes of arc from the edge of
stimuli of various sizes, the thresholds of the stim uli were independent
of variations in the separation angle.
It has been assumed by previous investigators that foveal receptors
are uniformly sensitivej i.e ., that different regions in the fovea have
essentially the same threshold when stimulated by a point stimulus. Explora­
tory data of Stiles (22) and Blackwell (4) indicate that this is not true
for near-monochromatic stim uli of certain wavelengths. The purpose of
Experiment II is to determine the threshold at different locations of the
fovea for the predominately 526 millimicron stimulus us ed in the present
investigation. The null hypothesis is that the fovea is everywhere equally
sensitive to a small source of light. No reason was found to reject the
null hypothesis.
The main part of the investigation is reported as Experiment III,
which consists of a determination of the area effect itse lf. Vfe have deter­
mined the threshold of fourteen circular areas varying in diamete r from0.707
to 64 minutes in angular subtense, each radius being 2 times the next
3
smaller radius. The radii are chosen to form a geometric progression so
that they lie at equal intervals on a logarithmic scale. Larger circles
would fa ll outside the rod-free area of the fovea. Due to the extensive
and uneven bipolar and ganglionic connections of the rods, and due to the
marked difference between "rod" and "cone" luminosity curves, it would be
impossible to coordinate with any degree of confidence data from areas
stim ulating only cones with data from areas stimulating both rods and cones.
The null hypothesis in Experiment III is that there is no area
effect which is not the result of optical effects. Previous data has been
complicated by the presence of chromatic aberration, eye oscillations,
doubtful fixation, etc., which alone or in combination could conceivably
account for previous evidence of the area effect. Our data, however, s till
lead to rejection of the null hypothesis.
Let us establish some terminological conventions. Reduction in
threshold by increase in a continuous stimulus area having a single border
w ill be called the "area effect." (An annulus has two borders; two
separate stim uli are not "continuous.") Change in threshold of an entire
stimulus configuration by a change in brightness of part of that configura­
tion w ill be called "spatial summation" or "spatial inhibition," accordingly,
as threshold is decreased or increased. The area effect is a special case
of spatial summation. The non-optical causes of spatial summation or in­
hibition, whether photochemical, neural or other, w ill be called "interactiony
The physiological effect of a stimulus, in the cortex or visual pathway,
w ill be called "excitation."
I
II. EXPERIMENTAL PROCEDURES
A. STIMULUS CONDITIONS
We may state as follows the basic criterion to be met by the
stimulus conditions of this investigation: optical effects are to be
made constant, minimal, or calculable.
Monocular presentation was selected, being simpler than bin­
ocular in these respects: two eyes might not focus the same or have
the same physiological or optical properties, and the exact effect
on thresholds of geniculate or cortical interaction between the two
visual pathways is not known.
An a rtific ia l pupil 0.089 in. (2.26 mm.) was used in order
to reduce optical blur due to spherical aberration and to reduce the
influence of the Stiles-Crawford effect (light striking the retina
obliquely has a reduced excitatory effect).
Background brightness was zero, so that a ll the sensory
mechanisms may be presumed to be available for stim ulation.
Fixation distance was 10.25 feet, a distance at which most
subjects fixate comfortably.
Exposure time was 0.001 second, which was selected in order
to minimize the effect of very small, rapid eye oscillations reported
by Adler and Fliegelman (2). Since the oscillations have a frequency
of 50-60 per second, the exposure time utilized eliminates most of
the smearing of the stimulus image on the retina due to such oscilla­
tions. Shorter exposure was not feasible because of the lim ited light
output available for the test stimulus. For a ll stimulus areas, the
exposure time employed is below the critical duration, that is, the
5
product of time and threshold intensity is constant. It is essential
that this condition be met so that the area effect w ill not be dis­
torted by time and intensity relations.
Several factors influenced the choice of chromaticity of the
test stimulus. White has been customarily used, but chromatic aber­
ration makes it very difficult to calculate the distribution of white
stimulus light on the retina. Since for this reason a monochromatic
stimulus is indicated, our first choice was a long-v/avelength red,
which may be presumed to "activate" only one "receptor system." The
eye is quite insensitive at these wavelengths, however, and our light
source was found not to be intense enough to reach threshold bright­
ness under these conditions. The next choice was a middle-wavelength
stimulus which would not only "activate" principally the "green
receptor system," but would also have high luminosity. The stimulus
finally settled on had peak luminosity at 526 m/(, with half the
luminous energy within an interval of 10.2 ny*. The spectral char­
acteristics are shown in Table 1 and Figure 1. Visual effect is here
defined to be equivalent to luminosity. The transmission of the
stimulus filte r w ill be discussed in part D, below.
With one exception, occurring in Experiment II, orientation
lights were arranged in a diamond about the test spot. Thus, the
observer sees one orientation light directly above ("North") of the
spot, and the three remaining orientation lights are East, Yfest and
South of the spot, respectively. As remarked above, Experiment I
was performed to determine the exact location of the orientation
lights. On the basis of Experiment I, it was decided to place the
orientation lights 33' visual subtense from the nearest edge of the